The present invention relates generally to the collection and transmission of power, and is particularly concerned with the collection of solar power and its transmission in the form of a microwave beam.
Electric power supplies for most of the earth's inhabitants are currently provided by heat engines (steam turbines or the like) fueled by combustion of fossil fuels (coal, oil and/or natural gas). Progressive exhaustion of readily accessible fossil fuel sources, particularly oil and gas, has led to higher prices, shortages, and prospects for even more expensive and inaccessible fossil fuel supplies in the future.
Renewable energy resources including direct and indirect solar energy systems, the latter including hydroelectric, wind, ocean thermal, photosynthetic fuels (biomass), production of hydrogen and others currently furnish an important but minor fraction of the earth's electric energy. These resources should eventually furnish virtually the entire energy supply unless nuclear fusion energy is rendered economically and technically feasible in the future.
Most of the direct and indirect solar power systems are subject to geographic and climatological limitations. The solar power systems are also subject to variable and unpredictable changes or interruptions of output power. This has necessitated the use of reserve power systems (usually fossil fuel fired) or the use of restrictions on usage to only portions of the daily or long-term power usage cycles. These techniques unfavorably affect the economics of renewable energy use.
Despite these problems, ongoing development work is proceeding for making direct use of solar photovoltaic conversion systems, particularly for locations in latitudes with sunny climates. Current and near-term projected costs of conversion elements are far too high to be competitive with fossil fuel systems, even in the absence of power storage systems associated therewith. Coupled with this is the fact that the most advantageous applications of photoconversion are in economically underdeveloped and isolated regions on earth. There is hope that improvements in engineering and manufacturing methods may lead to systems which are cost competitive with future fossil fuel power systems.
There have been proposals in the past to collect solar power by means of earth orbiting satellites carrying solar collectors, and then to beam the power to suitable rectenna or receiver structures on earth, where the power is stored and distributed to users. Because of the many technological problems involved, none of these systems have yet been put into effect.
One proposal for such a system is described in U.S. Pat. No. 3,781,647 of Glaser, in which an orbiting satellite collects sunlight, converts it into electrical energy, converts the electrical energy to microwaves and beams the microwave power to earth from a planar phased array of microwave transmitters. One problem with this system is that it operates in the so-called "far field" or divergent area of the microwave beam, as is also the case in virtually all radar and high gain communications systems. The radiating near and far field regions of a wave field propagating from a radiating aperture (e.g. antenna) structure are separated by a near field transition length which is approximately equal to L=D.sup.2 /w where D is the aperture diameter and w is the wavelength. In the radiating near field or Fresnel region (L.sub.n &lt;L), the maximum optical path difference from a field point to the distributed points of the aperture plane exceeds several wavelengths regardless of whether the field point is on or off axis of the system. Under such conditions, the radiated intensity is effectively averaged or smoothed for lateral displacement within the geometric optic beam and rapidly attenuates beyond the beam edge. Thus a beam is produced which closely approximates the geometric optics limit. In contrast, the radiating far field (Fraunhofer) region is divergent and has a maximum optical path difference of less than a wavelength between an axial field point and all points of the aperture plane. The radiated intensity decreases with lateral excursion from the beam center (with or without undulations depending on details of the aperture excitation.
Where an antenna system is used to transmit microwave power from a satellite, as taught by Glaser, the size of the antenna is limited, R is fixed, and the transition length L will be of the order of 10,000 km (for D=1 km and w=10 cm). Since geosynchronous satellites orbit at 36,000 km, the operating distance is more than three times the near field transition length for the above case. Thus the coherent microwave power of the beam will extend in a lobe-like Fraunhofer diffraction pattern across the earth. Because of the constraints on antenna sizes and operating frequencies for which sufficient atmosphere transparency exists, it would not be practical to operate in the near field in the Glaser system. The minimum beam diameter at the earth receiver would be about 3.6 km, and the approximate beam diameter would be 10 km. Thus the receiver must be very large in order to receive a substantial portion of the power adding considerable expense to the system.
While the Glaser system has certain merit in terms of providing the earth with needed electrical energy, there are major problems associated with the development of such a system. Primarily these considerations are both technological and economic. Technologically, there are many unanswered questions with regard to the feasibility and long term reliability of such terrestrial and space borne power generating systems. There are environmental problems restricting the total power which may be transmitted in this way. Such systems are also extremely expensive.